My mom wants to return a cat she got from a foster home where she was mistreated and kept in a cage what to do?

Recently I asked my mom to adopt a local cat who was fostered by some people in a foster home. She was mistreated and abused, kept in a cage all day long I think they only let her out of the cage once a day and the rest of the time she was in the cage. She had no freedom whatsoever at that fosetr home also the foster mom of the cat was feeding a ridiculosly low portion of food to this poor cat. Apparently the foster mom gave her only one can of wet food a day (half in the morning the other half at night) and the cat came to us so skinny the cat is ridicuosuly skinny even though she's already a year old. And she eats so much here she asks for food like very often I think because she was very hungry before where she was at the foster place for cats. Even though she eats a lot when she's now here she's still very skinny. My mom now thinks it was a mistake to get this cat because she has done some mischeif in our apartment so she is thinking of returning her to the foster home. The cat has peed on my bed, my grandma's bed and she did a bowel movement in my daughters bed. But however the reason she did that is cause she was kept in the rooms for a long time and no one knew and she had to go and her liter was not there so it's not her fault what was the cat supposed to do if she was stuck in a room for a long time without her liter box hold her bowels? I have no idea why my mom is blaming the cat for what she did.It's not the cat's fault she just had to go to the bathroom and was stuck in a room and no one knew so she went in the bed I mean she couldn't hold it so what was she supposed to do in this case? My mom does not like the cat's behavior and wants to return the cat back to where she got her. However I feel sorry for the cat and I know she did not have a good life there and do not wish to return her. If she returns there they'll lock her back into a cage and mistreat her and feed her poorly and I do not wish this kind of life for the cat. She is also already used to our environement and I am afraid if she is put back to where she came from it will be very stressful for her and she might have a heart attack and die. Really this cat has been through enough as it is and now she's lucky she at least got a home and I do not want her to loose this. If we never adopted this cat I doubt anyone else would. She's lucky, if not for me finding her on pet finder and adopting her she either would have stayed in a cage for the rest of her life or if no one took her they would eventually eutanize her and put her to sleep so I kind of saved the cats life but my mom said that she wants to give her away back because she peed in the bed, also she says my daughter fears her a little. Also she said she does not like the cats behavior, the cat jumps everywhere, on kitchen tables and all that and she tears up furniture and couches with her claws and teeth, my mom does not like this and wants to return this cat and get a kitten instead. She also wants a kitten but fears this cat will not get along with a kitten so she wants to get rid of the cat because she wants a kitten and fears them two wont get along, I however want to keep the cat, I am feeling bad because she might die if they lock her up again she got used to a home and if thats taken away from her and shes locked back up in the cage she might get so stressed and have a massive heart attack. Also she has a bond with me and she rubs against me and purrs she really likes me a lot and I am afraid she'll have a heart attack if she is taken away from me. What to do? How to convince my mom to keep the cat?